Contact sensitive device

ABSTRACT

Bending wave vibration is used to calculate information relating to a contact on a contact sensitive device. The contact sensitive device has a member capable of supporting bending waves, and a device attached to the member measures bending wave propagation in the member to determine a measured bending wave signal. The measured bending wave signal is processed to calculate information relating to the contact. The contact sensitive device may comprise a transparent touch sensitive plate mounted in front of a display device.

This application is a divisional of application Ser. No. 09/746,405,filed Dec. 26, 2000, which application claims the benefit under 35U.S.C. §119(e) of provisional application No. 60/171,603, filed Dec. 23,1999 and provisional application No. 60/242,618, filed Oct. 23, 2000,all of which applications are incorporated by reference herein.

TECHNICAL FIELD

The invention relates to contact sensitive devices, e.g. devices thatdetect and process information based on the location of a transientcontact made on a panel or screen.

BACKGROUND ART

Visual displays often include some form of touch sensitive screen. Thisis becoming more common with the emergence of the next generation ofportable multimedia devices such as palmtop computers. The mostestablished technology using waves to detect contact is Surface AcousticWave (SAW), which generates high frequency waves on the surface of aglass screen, and their attenuation by the contact of a finger is usedto detect the touch location. This technique is “time-of-flight,” wherethe time for the disturbance to reach one or more sensors is used todetect the location. Such an approach is possible when the mediumbehaves in a non-dispersive manner, i.e. the velocity of the waves doesnot vary significantly over the frequency range of interest.

SUMMARY OF THE INVENTION

According to the invention, there is provided a method of determininginformation relating to a contact on a contact sensitive devicecomprising the steps of:

-   -   providing a member capable of supporting bending wave vibration,    -   contacting the member at a discrete location to produce a change        in bending wave vibration in the member,    -   measuring the changed bending wave vibration in the member to        determine a measured bending wave signal, and processing the        measured bending wave signal to calculate information relating        to the contact.

The contact may be in the form of a touch from a stylus or a finger. Thestylus may be in the form of a hand-held pen.

The information calculated may be the location of the contact or may beother information, e.g. pressure or size of the contact. The informationrelating to the contact may be calculated in a central processor.

The bending wave propagation may be measured by at least one sensorwhich may be mounted at or spaced from an edge of the member. The sensormay be in the form of a sensing transducer which may convert bendingwave vibration into an analogue input signal. There may be more than onesensor.

By bending wave vibration it is meant an excitation, for example by thecontact, which imparts some out of plane displacement to the member.Many materials bend, some with pure bending with a perfect square rootdispersion relation and some with a mixture of pure and shear bending.The dispersion relation describes the dependence of the in-planevelocity of the waves on the frequency of the waves. The relativemagnitude of the vibration is determined by material properties of themember and the frequency of excitation.

Bending waves are dispersive, i.e. the bending wave velocity isdependent on frequency. This property makes any “time-of-flight”approach inappropriate, as the signature of the disturbanceprogressively spreads out in time. Accordingly, the method furthercomprises the step of applying a correction to convert the measuredbending wave signal to a propagation signal from a non-dispersive wavesource. Once the correction is applied, techniques used in the fields ofradar and sonar may be applied to detect the location of the contact.

One significant advantage of using bending wave propagation is thatbending waves are bulk waves, which involve the movement of the wholemember, and not just the surface. In contrast, most of the alternativetouch sensing technologies rely on surface effects and as such arevulnerable to surface damage. Accordingly, a contact sensitive deviceusing bending waves should be more robust and less sensitive to surfacescratches, etc.

Applying the correction may be the first step in processing the bendingwave signal. The correction applied is preferably based on thedispersion relation of the material of the member supporting the bendingwaves. This dispersion relation may be modelled by using the bendingwave equation in combination with known physical parameters of thematerial of the member. Alternatively, the dispersion relation may bemeasured by using a laser vibrometer to create an image of the vibrationpattern in the member for a number of given frequencies to give thedispersion relation in the frequency range of interest.

The measuring of bending wave propagation may be done by continuallysampling the motion in the member. By comparing the measured bendingwave signal with a reference signal, for example the signal before acontact is made, it may be possible to identify when contact is made.The magnitude or other characteristics of the signal may be compared.Once contact has been made, the measured bending wave signal may berecorded and may then be processed.

The member may be in the form of a plate or panel. The member may betransparent or alternatively non-transparent, for example having aprinted pattern. The member may have uniform thickness. Alternatively,the member may have a more complex shape, for example a curved surfaceand/or variable thickness. Provided it is possible for bending waves totravel from the contact position to one of the sensors (by whatevercomplex path), the method may be adapted for complex shaped members byproviding an adaptive algorithm such as a neural net to decipher thecontact location from the bending wave signal received by the sensor. Itmay be necessary to have several sensors.

The method may involve purely passive sensing, in other words, thechange in bending wave vibration in the member induced by the contactmay be the excitation to bending wave vibration in the member. In otherwords, there is no other source of bending wave vibration for a passivesensor. The position of the contact may be calculated by recording thetime of arrival of an impulse at each sensor, comparing the times todetermine the relative distances of each sensor from the origin of theimpulse and intersecting the relative distances to give the position ofthe contact. The bending wave vibration and hence the measured bendingwave signal may be generated by an initial impact or by frictionalmovement of the contact. There may be a minimum of three sensors.

Increasing the number of sensors used to detect the contact or contactlocation provides extra information and thus may provide a more accuratedetection. Alternatively or additionally, the bending wave signalreceived at each sensor may be analysed over a longer period of timesuch that not only the direct signal, i.e. the signal when the impulsefirst reaches the transducer, is measured but also the reflections fromthe edges of the member. This approach is similar to adding mirroredversions of the or each existing sensor. Using this scheme, the extrainformation obtained may be used to provide greater accuracy or reducethe number of sensors.

After calculating the location of the contact, the measured bending wavesignal may be further processed to determine additional informationregarding the contact. The movement of a stylus on the member maygenerate a continuous signal which is affected by the location, pressureand speed of the stylus on the member. Continuous time data which may bederived from the continuous signal may be used to derive additionaluseful information in a variety of applications.

One application may be signature recognition which is a subset of themore general task of pattern recognition. Applications such as these,where patterns are drawn from complex data, benefit greatly from theextra independent information present in the continuous time data. Themethod may thus further comprise the step of implementing a neural netfor processing continuous time data. The neural net may be trained by aset of examples, for example, a set of signatures written by aparticular subject or a set generated from a knowledge of the typicalvariance caused by the human process of writing.

A fundamental property of a neural net is that the more independentinformation is available, the greater the accuracy of the conclusionsdrawn. Much of the information available in the continuous time data iscompletely independent from the position information, since it isconnected to the velocity and pressure of the stylus on the surface ofthe member. Therefore the extra information increases the potential foraccurate signature recognition. The method may further include thetraining of a second neural net with examples of time responses forsignatures. Additional improvement may be achievable with training usingfurther examples, either generated by the user or from knowledge of theexpected variations in pressure and velocity.

Alternatively, the continuous time data may be used in handwritingrecognition, the detection of a “double-click” or the detection of thestrength of a contact, e.g. how hard a click. Both detection of“double-click” and click strength may be achieved with the image of theimpulse shape in the continuous-time data. It may be possible to use aslower position sampling rate than other more conventional technology.

In contrast, conventionally the detection of a contact, be it pen,finger, etc., is performed at a pre-determined sample rate and theinformation concerning the contact location is built up from a set ofpoints. There is no continuous time information and thus many of theapplications described above may not be performed or may be performedless satisfactorily.

A measurement of the frequency content of the measured bending wavesignal may be used to determine the contact type since thecharacteristic frequencies generated by each type of stylus differ. Forexample, a hard stylus will generate higher frequencies than a softfinger. Thus, a contact sensitive device for use with a hand-heldpen-input device may be set up so as not to be triggered if the hand ofthe operator touches the contact sensitive device.

The differences in the frequency generated by different types of styliimply a difference in the absolute spatial resolution achievable; thehigher frequency translates to a greater resolution. However, theresolution difference often coincides with the requirements for thecontact in question. For example, the spatial resolution required for aninput by a finger is usually less than the spatial resolution expectedfor a sharp-tipped stylus.

The frequencies generated by the contact are relatively low, i.e.generally audio frequencies rather than ultrasonic. Consequently, themember is preferably capable of supporting bending wave vibration in theaudio range. Thus, a member similar to those used as an acousticradiator in a loudspeaker may also be used to act as a contact sensitivedevice.

The contact sensitive device may further comprise an emitting transducermounted on the member to generate bending wave vibration in the memberto probe for information relating to the contact. The member may thus bean acoustic radiator and bending wave vibration in the member may beused to generate an acoustic output. Such vibration may be regarded as anoise signal, although there are other types of noise signal which mayeffect the passive sensing. When there is an external noise signal, themethod may further comprise techniques to isolate the noise signal fromthe signal generated by the contact, for example:

-   1) Prediction filtering which predicts the response of the noise    signal over a short time scale. Differences from the predicted value    are more likely to be generated by a contact than by the emitting    transducers.-   2) Modelling the noise signal using a continuous logging of the    audio signal produced, together with knowledge of the transfer    function from the emitting transducer to the sensor. This allows a    more accurate prediction of the noise signal than prediction    filtering.-   3) Using the multiple sensors to determine the location of the    emitting transducer in the same manner as used to locate the contact    (for example, intersection method). This information should    facilitate the separation of the bending waves generated by the    emitting transducer from the bending waves generated by the contact.

Alternatively, the noise signal may be used as an active probe of acontact in the member. Thus, the method may further comprise generatingbending wave vibration in the member so that there is active sensing, inother words, sensing which relies not on the generation of waves by thecontact but on the response of waves already present in the member to amechanical constraint caused by the contact.

The bending waves in the member may be generated by a stimulus signalfrom a transducer mounted on the member. The transducer may have dualfunctionality, namely acting as an emitting transducer and a sensor.Alternatively, there may be an emitting transducer and at least onesensor mounted on the member.

The effect of the contact may be reflective, absorbing, or a combinationof the two. For reflection, an emitting transducer generates bendingwaves, which are reflected by the contact and detected either by thesame transducer or a separate sensor. The signal, either a time orfrequency response, may then be processed with the material dispersionrelation information to yield the distance travelled from the emittingtransducer or source to the sensor via the contact.

One single measurement may be sufficient to differentiate between twocontact locations which are a substantial distance apart. However, moreinformation may be required to determine the contact location moreaccurately. This may be achieved by sensing the reflection with multiplesensors, where the stimulus signal may emanate from the emittingtransducer or from a different source for some or all of the sensors.Either way, each sensor gives an independent measurement of the contactlocation, which may be combined to give a progressively more accuratecontact location with increasing transducer number.

An alternative way to increase the location accuracy may be to measurethe bending wave vibration in the member over a longer time, thusincreasing the information in each measurement. In terms of a frequencyresponse, this may correspond to a greater frequency resolution. Theextended signal may also contain information concerning both direct andindirect reflection from the contact. Indirect reflection is a signalwhich arrives at the sensor from the contact via one or more boundaryreflections. This method may be regarded as equivalent to adding furthersensors at the mirror locations of the initial sensor, and may beemployed to determine an accurate contact location with only onecombined source/sensing transducer.

A self-measuring scheme may be incorporated into the contact sensitivedevice to measure the material dispersion relation in the member. Whenno contact is applied the boundary reflections are still present, whichfor a regular shape are manifest as strong reflections corresponding tothe distances to each boundary. For a specific implementation, theemitting transducer, sensor and boundary locations are known which givesa set of known reference points. A smooth function representing thematerial dispersion relation may then be optimised to warp the frequencyaxis such that the periodicities corresponding to these reference pointsare restored. Further optimisation may be performed if required byadding other known reference points such as a contact in apre-determined place.

This scheme allows an implementation of the active sensing techniquewithout prior knowledge of the material dispersion relation.Alternatively it may be used to fine tune a correction for the smallmanufacturing tolerances present in the panel properties, or variationsdue to heat, humidity, etc.

Pure absorption requires a different implementation as compared to ascheme based on reflection. Thus the method may comprise implementing a“ray tracing scheme,” where the effect of the contact is to interrupt awave incident on one or more of the sensors. A wave incident on a sensormay be created by direct excitation, e.g. by one or more emittingtransducers at an opposed location, or by indirect excitation from oneor more boundary reflections. For indirect excitation, the emittingtransducer may be located at any position, including a position adjacentto the sensor. Furthermore, indirect excitation allows detection of anabsorbing contact from a single transducer, which acts as the source andthe sensor of the boundary reflections.

Interruption of the incident wave may also result in diffraction aboutthe absorption point. The effect of diffraction makes the absorptiveapproach sensitive to a much wider area than is the case for pure raytracing. The contact location may be outside a direct path of thebending wave incident on the sensor and may still affect the signalreceived by the sensor. The information obtained by absorption may be ina more complex form than that for a reflecting contact. Consequently amore intelligent detection algorithm may be required, such as a neuralnet.

The stimulus signal generated by the transducer preferably has goodnoise rejection, and preferably does not have an audibly damaging oracoustically obvious effect. Thus, the stimulus signal may have a verysmall amplitude or may be similar to noise. For the latter, a particularcorrelation may be hidden in the noise for the calculations to latchonto. Alternatively, the stimulus signal may be made inaudible, i.e.ultrasonic by increasing the frequency above 20 kHz. This has theadvantage that a large signal amplitude can be used and the highfrequency translates into a high spatial resolution. However, the membermust be capable of supporting such an ultrasonic signal. Many materialsare suitable, for example, glass, crystal polystyrene.

The stimulus signal may be chosen from any one of the following signals:

-   1. Pulsed excitation—note this does suffer from poor noise rejection    and audibility, if it has sufficient amplitude.-   2. Band limited noise—this signal is less audibly damaging than most    in any given frequency band and has the advantage that it may be    tuned to the most suitable frequency band. In addition it may be    made ultrasonic.-   3. Steady state sine waves—these give good signal to noise but are    extremely audible when in the audio band. Improvements are to place    the frequency outside the audio band or use multiple closely spaced    sines with random relative phase, thus making the signal audibly    more noise-like. This is one example of a signal that is audibly    noise-like, but has a hidden correlation that improves the signal to    noise level. Another example of such a trace is an MLS (Maximum    Length Sequence) signal.-   4. A chirp signal—this is a widely used signal to determine a    frequency response of a system over broad range of frequencies.    However this may be practical only at ultrasonic frequencies, where    it is not audible.-   5. An audio signal—this may be fed into the transducers when the    member is being used as an acoustic radiator for a loudspeaker. In    this case there is no problem with the stimulus signal having an    audibly damaging effect, as it is the very signal responsible for    the intended audio output.

When a sensor and an emitting transducer are close together or the sametransducer, a background signal produced by the emitting transducer isgenerally much greater than the signal of interest associated with thecontact. This may introduce problems which may be alleviated in a numberof ways. For example, for a pulsed excitation signal, the measurement atthe sensor may be gated so that measurement starts after an outgoingwave produced by the emitting transducer has progressed further than thesensor. However, extended time stimulus signals are more common thanpulsed excitation signals since the latter has poor noise rejectionproperties.

For an extended time stimulus signal there are mechanical or othertechniques which may be used to improve the relative magnitude of thecontact signature, for example:

-   1) Placing the sensor at approximately ¼ wavelength from the    emitting transducer so that the magnitude of the outgoing wave    detected at the sensor location is minimised. This technique may be    used if the contact signal is limited to a relatively narrow range    of frequencies.-   2) Locating the emitting transducer and the sensor at one drive    point and designing the emitting transducer and the sensor to couple    into orthogonal physical properties. For example, a bender    transducer and an inertially coupled transducer may be located at    the same point. An outgoing wave generated by either transducer is    not detected by the other. However, a secondary wave which is either    reflected from the contact or boundaries is detected, maximising its    relative magnitude.-   3) Addressing the problem in the electrical domain. A measurement of    the frequency response may be achieved with a swept sine wave and a    demodulation stage. The outgoing wave from the emitting transducer    produces a large background value of the frequency response upon    which the fine structure due to smaller reflections from the contact    is superposed. After demodulation (e.g. by a chirp demodulation    circuit) the output may be a small ripple on a large smoothly    varying background. Consequently, when this output is passed through    a high pass filter the pertinent fine structure may be emphasised    relative to the large background.-   4) Digitising the measured signal with sufficient accuracy so that    it is sensitive to the fine structure on top of the large    background. The fine structure may then be emphasised with filtering    in the digital domain.

Depending on the use of the transducer it may either be a two, three, orfour terminal device. Two terminal devices may be used as sensors oremitting transducers separately. Alternatively they may be used as dualfunction transducers, where a sensing function is determined from theimpedance of the device. Three and four terminal devices use a separatetransducer as sensor and emitting transducer. For a three-terminaldevice the sensor and emitting transducer share a common electrode,whereas the sensor and emitting transducer are electrically isolated inthe four-terminal device.

The or each emitting transducer or sensor may be a bender transducerwhich is bonded directly to the member, for example a piezoelectrictransducer. The bender transducers are generally directional, which maybe advantageous in some applications. The directivity achieved isdetermined by their physical shape and may therefore be tunedaccordingly. Additional advantages include a high conversion efficiency,low cost, and considerable robustness.

Alternatively, the or each emitting transducer or sensor may be aninertial transducer which is coupled to the member at a single point.The inertial transducer may be either electrodynamic or piezoelectric.Inertial transducers are generally omni-directional, provided thecontact point is small compared to the bending wavelength in the memberat the frequency of interest.

The transducers and/or sensors may be placed with a relatively equalspacing around the edge or on the surface of the member subject to thespecific topology of the application.

It may be possible to use audio transducers which are already in placeas sensing and/or emitting transducers. This implementation may add thefacility for a touch screen with the minimum of extra hardware. However,if this approach is not possible then small piezo elements might provethe most suitable transducers, as these are particularly suited to theultrasonic frequencies which may be used for active sensing.

According to another aspect of the invention, there is provided acontact sensitive device comprising a member capable of supportingbending wave vibration, at least one sensor coupled to the member formeasuring bending wave vibration in the member, and a processoroperatively coupled to the sensor for processing information relating toa contact made on a surface on the member from the change in bendingwave vibration in the member produced by the contact and measured by thesensor.

The contact sensitive device may be a passive sensor where bending wavevibration in the member is only excited by the contact and not by anyother source. Alternatively, the contact sensitive device may be anactive sensor. The contact sensitive device may thus further comprise anemitting transducer for exciting bending wave vibration in the member toprobe for information relating to the contact. Information relating tothe contact is calculated by comparing the response of waves generatedby the emitting transducer to a mechanical constraint caused by thecontact.

The member may be capable of supporting bending waves in the audiorange. The contact sensitive device may thus be a loudspeaker such thatan acoustic radiator of the loudspeaker acts as the member of thecontact sensitive device and an exciter mounted on the acoustic radiatorto excite bending wave vibration in the acoustic radiator to produce anacoustic output acts as the emitting transducer of the contact sensitivedevice.

The contact sensitive device may further comprise display screen, e.g.for presenting information related to the contact which is calculated bythe processor. Thus, according to a further aspect of the presentinvention, there is provided a display screen which is a contactsensitive device. The display screen may be a liquid crystal displayscreen comprising liquid crystals which may be used to excite and/orsense bending waves. The screen may be capable of supporting bendingwaves over a broad frequency range. Direct contact to the screen maytrigger the contact sensitive device. This application therefore affordsthe possibility to make a standard LCD screen touch sensitive with noadditional mechanical parts.

Since the method may be adapted to complex shapes, a contact sensitivedevice according to the invention may be included in a mobile phone, alaptop computer or a personal data assistant. For example, the keypadconventionally fitted to a mobile phone may be replaced by a continuousmoulding which is touch sensitive according to the present invention.This approach may decrease costs and provide an extended area for use inaudio applications. In a laptop, the touchpad which functions as a mousecontroller may be replaced by a continuous moulding which is a contactsensitive device according to the invention. The moulding may beimplemented as a mouse controller or other alternatives, e.g. akeyboard.

The advantages of the bending wave contact sensitive device and methodcompared to other technologies are:

-   1) A more versatile technology which is sensitive to both location    and pressure of the contact;-   2) A cheaper form of contact sensitive device since there is no    requirement for an array of transparent contacts or a complex sensor    of a magnetic tip, etc.;-   3) The device is readily scaleable in size and spatial sensitivity    by control of the material parameters of the member; and-   4) By using a dual functioning member, good quality sound may be    achieved within tight spatial and weight constraints.

BRIEF DESCRIPTION OF THE DRAWING

Examples that embody the best mode for carrying out the invention aredescribed in detail below and diagrammatically illustrated in theaccompanying drawing, in which:

FIG. 1 is a schematic front view of a touch sensitive device accordingto the present invention;

FIGS. 2 a and 2 b are schematic perspective views of a bending wavedevice before and after contact is applied;

FIG. 3 is a schematic perspective view of a first example of a deviceincorporating passive touch sensing according to a first embodiment ofthe present invention;

FIG. 4 is a schematic perspective view of a second example of a deviceincorporating passive touch sensing according to the first embodiment ofthe present invention;

FIG. 5 is a block diagram of a processing algorithm for passive sensingaccording to the first embodiment of the present invention;

FIG. 6 is a schematic perspective view of a first example of a deviceincorporating active touch sensing according to a second embodiment ofthe present invention;

FIG. 7 is a schematic perspective view of a second example of a deviceincorporating active touch sensing according to the second embodiment ofthe present invention;

FIG. 8 is a block diagram of an implementation topology of the presentinvention;

FIG. 9 is a block diagram of a processing algorithm for active sensingaccording to the second embodiment of the present invention; and

FIGS. 10 a to 10 d are graphic illustrations of a method of dispersioncorrection.

DETAILED DESCRIPTION

FIG. 1 shows a contact sensitive device (10) comprising a transparenttouch sensitive plate (12) mounted in front of a display device (14).The display device (14) may be in the form of a television, a computerscreen or other visual display device. A stylus (18) in the form of apen is used for writing text (20) or other matter on the touch sensitiveplate (12).

The transparent touch sensitive plate (12) is also an acoustic devicecapable of supporting bending wave vibration. Three transducers (16) aremounted on the plate (12). At least two of the transducers (16) aresensing transducers or sensors and are thus sensitive to and monitorbending wave vibration in the plate. The third transducer (16) may alsobe a sensing transducer so that the system corresponds to the passivecontact sensitive device of FIG. 3 or FIG. 4.

Alternatively, the third transducer may be an emitting transducer forexciting bending wave vibration in the plate so that the systemcorresponds to the active embodiment of FIGS. 6 and 7, which may act asa combined loudspeaker and contact sensitive device.

FIGS. 2 a and 2 b illustrate the general principles of a contactsensitive device (22) using bending wave vibration as the sensingelement. The contact sensitive device (22) comprises a panel (24)capable of supporting bending wave vibration and a sensing transducer(26) mounted on the panel (24) to sense bending wave vibration in thepanel (24) at the point where the sensing transducer (26) is mounted.FIG. 2 a shows the vibration pattern (28) of bending wave vibration, inthis case the normal uninterrupted vibration pattern, e.g. that ofsteady state at a given frequency, or a transient pulse.

In FIG. 2 b, contact has been made to the panel (24) at contact point(30) and the pattern of vibration is altered. Contact may alter thevibration pattern (28) either by disturbing the path of bending wavesalready in the panel (24) or by generating new bending waves whichemanate from the contact point (30). The change in vibration pattern(28) is sensed by the sensing transducer (26). Information relating tothe contact may be determined from the readings of the sensingtransducer, for example, by a first processing unit. The information maybe relayed to a second processing unit which outputs the information ona display screen. The information may include details of the locationand pressure profile of the contact impulse, for example:

-   1) The x,y co-ordinates of the contact.-   2) The characteristic size of the contact, e.g. 1 mm corresponds to    a pen or stylus, 1 cm corresponds to a finger.-   3) Profile of pressure of contact as a function of time.

FIGS. 3 and 4 are more detailed illustrations of two contact sensitivedevices (32, 33). Each of the contact sensitive devices (32, 33)comprises a panel (24) capable of supporting bending wave vibration andthree sensing transducers (26) for sensing bending wave vibration attheir respective mounting points. The vibration pattern (28) is createdwhen pressure is applied at a contact point (30). The devices may beconsidered to be passive contact sensitive devices since the devices donot comprise an emitting transducer. Thus the bending wave panelvibration in the panel is generated solely by the contact.

In a passive sensor an impulse in the body of the panel (24) starts abending wave travelling towards the edge of the panel (24). The bendingwave is detected by the three sensing transducers (26) mountedequidistantly around the edges as in FIG. 3, or by the three sensingtransducer mounted on a surface of the panel (24) but spaced from theedges of the panel (24) as in FIG. 4. The measured bending wave signalsare processed to determine the spatial origin and force profile of theapplied impulse.

FIG. 5 shows an algorithm for the processing of the bending waveinformation sensed at each sensing transducer (26) of FIG. 3 or FIG. 4.The algorithm comprises the following steps:

-   i) Optimise the signal at each sensing transducer to minimise    external unwanted signals. Linear prediction of the signal can be    used to predict and remove background noise.-   ii) Calculate the frequency response at each transducer.-   iii) (Optional) Add in information on the location of the contact    impulse if available from active sensing.-   iv) Add in material parameter information.-   v) Using the information available from steps (ii), (iii) and (iv),    correct for panel dispersion to give non-dispersive response.-   vi) Compute the inverse fft of the response at the contact time    giving the impulse shape at the contact point.-   vii) Output information detailing the impulse shape and location    information if required.

Advantages of passive sensing include:

-   1) the method encompasses more than one frequency and includes    sufficient frequency content required to image the impulse shape,    and-   2) as the method is passive the power requirements are minimal.

One disadvantage of passive sensing is that the frequency content of themeasured signal is limited by the frequency content of the impulse.Consequently the high frequency information is limited, which translatesinto a relatively long bending wavelength. The spatial resolution of thesignal is therefore limited.

FIGS. 6 and 7 are more detailed illustrations of alternative combinedtouch sensitive and audio devices (35, 37). The devices each comprise apanel (24) capable of supporting bending wave vibration and an emittingtransducer (31) for exciting bending wave vibration in the panel (24).The device (35) in FIG. 6 further comprises two additional sensingtransducers (26) for sensing bending wave vibration at their respectivemounting points, whereas the device (37) in FIG. 7 comprises oneadditional sensing transducer (26). The vibration pattern (28) isinterrupted when pressure is applied at a contact point (30). Thedevices may be considered to be active contact sensitive devices sincethe devices comprise an emitting transducer (31).

In FIG. 6 the sensing and emitting transducers (26, 31) are spacedequidistantly around the edges of the panel (24), whereas in FIG. 7 thesensing and emitting transducers (26,31) are distanced from the edges ofthe panel (24) and are mounted to a surface thereof. The transducers inFIG. 7 are spaced equally on the surface of the panel.

FIGS. 8 and 9 illustrate possible implementations of the active contactsensitive device. In FIG. 8, the central processor (34) outputs adigital output signal (36) which is converted by the digital to analogueconverter (DAC) (38) to an analogue output signal (40). The analogueoutput signal (40) is fed to an amplifier (42) which feeds an amplifiedanalogue output signal (44) to the emitting transducer (31). Theemitting transducer (31) emits bending wave excitation (46) whichexcites bending waves in the panel (48).

The bending waves in the panel (48) are sensed at sensing step (50) bytwo sensing transducers (26). The sensing transducers (26) convert thebending wave vibration into analogue input signals (52) which are fedinto an input analogue to digital converter (ADC) (54). The resultantdigital input signal (56) is transmitted to the central processor (34)from which information (58) relating to the location and profile of thecontact impulse is determined.

In FIG. 9, there is shown a method for determining the location of thecontact point. The steps are as follows and may be performed by thecentral processor shown in FIG. 8:

-   a) Measure frequency response at each sensing transducer.-   b) Correct for panel dispersion relation.-   c) Compute the fft to give the time response for a non-dispersive    medium.-   d) Compare the time response to a reference response, where there is    no external contact to the panel.-   e) Identify the reflections originating from the contact point.-   f) Perform echo location on the relevant reflections to identify    their origin.-   g) Output the information detailing the location of the contact.

Advantages of active sensing include:

-   1) As the technique measures the response to an external signal,    high frequency information is not limited and a high spatial    resolution is possible.-   2) The susceptibility to external noise can be greatly reduced. This    can be achieved by sensing the response in a frequency band where    the external noise is small, such as above the audible spectrum. An    alternative is to give the signal a particular correlation, enabling    its detection even when small compared to the background noise.

Disadvantages of active sensing include:

-   1) The technique is likely to be less sensitive to the profile of    the impulse than the passive scheme. However, more sophisticated    processing may improve this situation. For example, the greater the    pressure of a finger or pen the larger the degree of extra damping    likely to be introduced. This may be identified by a relative simple    extra processing of the data.-   2) The need for an external signal is likely to require more power    than the passive measurement. This drawback can be minimised by    making the exciting signal as small as possible. Also, when the    exciting signal is at high frequency piezo transducers may be    employed, which have the advantage of a very high efficiency.

In many applications the one single implementation of the bending wavecontact sensitive device may not be general enough to cope with allsituations. For example a passive sensor will work well when there is noaudio being played through the device. However, when loud music is beingplayed, an active sensor, either at frequencies out of the audio band orusing the musical signal as the stimulus, is more suited. Therefore acombination of more than one particular implementation may prove to bethe best solution. Furthermore, in the transition region between thepassive and active sensing there may be useful information obtainablefrom both techniques.

FIGS. 10 a to 10 d show the steps in one possible method of correctingto convert the measured bending wave signal to a propagation signal froma non-dispersive medium. FIG. 10 a is a graph of a dispersive impulseresponse showing response in arbitrary units against time. FIG. 10 b isa graph of a dispersive frequency response showing response in arbitraryunits against frequency. FIG. 10 c is a graph of a non-dispersivefrequency response showing response in arbitrary units againstfrequency. FIG. 10 d is a graph of a non-dispersive impulse responseshowing response in arbitrary units against time.

For pure plate bending, the wave speed is proportional to the squareroot of frequency, i.e. the high frequency component of any particularwave travels faster than the lower frequency components. FIG. 10 a showsan impulse in an ideal medium with a square root dispersion relation anddemonstrates that a dispersive medium does not preserve the wave shapeof an impulse. The outgoing wave (60) is evident at time t=0 and theecho signal (62) is spread out over time, which makes a determination ofan exact contact position problematic.

A periodic variation of the frequency response is characteristic of areflection, and is often referred to as comb filtering. Physically, theperiodic variation in the frequency response derives from the number ofwavelengths that fit between the source and the reflector. As thefrequency is increased and the number of wavelengths fitting in thisspace increases, the interference of the reflected wave with theoutgoing wave oscillates between constructive and destructive.

Calculating the Fourier transform of the dispersive impulse response ofFIG. 10 a produces the frequency response shown in FIG. 10 b. Thefrequency response is non-periodic and the periodic variation withwavelength translates to a variation in frequency that gets slower withincreasing frequency. This is a consequence of the square rootdispersion in which the wavelength is proportional to the square root ofthe inverse of frequency. The effect of the panel on the frequencyresponse is therefore to stretch the response as a function of frequencyaccording to the panel dispersion. Consequently, a correction for thepanel dispersion may be applied by applying the inverse stretch in thefrequency domain, thus restoring the periodicity present in thenon-dispersive case.

By warping the frequency axis with the inverse of the panel dispersion,FIG. 10 b may be transformed into the frequency response for thenon-dispersive case (FIG. 10 c) in which the frequency of excitation isproportional to the inverse of the wavelength. This simple relationshiptranslates the periodic variation with decreasing wavelength to aperiodic variation with increasing frequency as shown in FIG. 10 c.

Applying the inverse Fast Fourier Transform (fft) to the trace of FIG.10 c produces an impulse response shown in FIG. 10 d, which is correctedfor dispersion and where the clear reflection is restored. As is shownin FIG. 10 d, any particular wave shape of an impulse is preserved intime since the waves travelling in a non-dispersive medium have aconstant velocity of travel, independent of their frequency.Accordingly, the task of echo location is relatively straight-forward.The outgoing wave (66) is evident at time t=0, together with a clearreflection (68) at 4 ms. The reflection (68) has a magnitude which isapproximately one-quarter of the magnitude of the outgoing wave (66).

The invention thus provides a novel and advantageous contact sensitivedevice, and a contact sensitive device combined with a bending wavepanel acoustic device. Various modifications will be apparent to thoseskilled in the art without departing from the scope of the invention,which is define by the appended claims.

Incorporated herein by reference are UK priority application No.9930404.0, filed Dec. 23, 1999; U.S. provisional application No.60/171,603, filed Dec. 23, 1999; and U.S. provisional application No.60/242,618, filed Oct. 23, 2000.

1. A method of determining information relating to a contact on anactive contact sensitive device comprising the steps of: providing apanel-form member capable of supporting bending wave vibration;generating bending wave vibration in the member from one location on themember to probe for information relating to a contact; contacting themember at a discrete location to produce a change in the generatedbending wave vibration in the member; measuring the changed bending wavevibration in the member at two locations on the member to determine ameasured bending wave signal; and processing the measured bending wavesignal to calculate information relating to the contact includingapplying a correction to convert the measured bending wave signal to apropagation signal from a non-dispersive wave source.
 2. A methodaccording to claim 1, wherein the information relating to the contactcomprises location of the contact.
 3. A method according to claim 1,wherein the information relating to the contact comprises pressure ofthe contact.
 4. A method according to claim 1, wherein the informationrelating to the contact comprises size of the contact.
 5. A methodaccording to claim 1, wherein movement of the contact on the membergenerates a continuous signal which is affected by location, pressureand speed of the contact on the member, and continuous time data fromthe continuous signal is used to derive additional information relatingto the contact.
 6. A method according to claim 5, further comprisingusing a neural net for processing the continuous time data.
 7. A methodas recited in claim 1 wherein the information relating to the contactcomprises a contact type.
 8. A method according to claim 7, wherein thecontact type is selected from the group consisting of touch by a stylusand touch by a finger.
 9. A method according to claim 1, wherein themeasuring step comprises measuring frequency content of the measuredbending wave signal to determine a contact type.
 10. A method accordingto claim 1, further comprising comparing the measured bending wavesignal with a reference signal to identify when contact is made.
 11. Amethod according to claim 1, wherein the measuring step comprisesmeasuring the changed bending wave vibration at two edges of the member.12. An active contact sensitive device comprising: a member capable ofsupporting bending wave vibration and forming an acoustic radiator whenexcited; one or more exciters coupled to the member for exciting bendingwave vibration in the member to probe for information relating to acontact made on a surface of the member, and to cause the member toproduce an acoustic output; at least one sensor coupled to the memberfor measuring bending wave vibration in the member; and a processoroperatively coupled to the at least one sensor for processinginformation relating to the contact from a change in bending wavevibration in the member produced by the contact and measured by the atleast one sensor and for applying a correction to convert the measuredbending wave signal to a propagation signal from a non-dispersive wavesource.
 13. A contact sensitive device according to claim 12, wherein anexciter of the one or more exciters is operable to both excite bendingwave vibration in the member and to cause the member to produce anacoustic output.
 14. A contact sensitive device according to claim 12,wherein the one or more exciters comprise at least two exciters andseparate ones of the at least two exciters are operable to excitebending wave vibration and to cause the member to produce an acousticoutput.
 15. A contact sensitive device according to claim 14, whereinthe exciter to excite bending wave vibration is operable to excite thebending wave vibration outside of audio frequencies.
 16. A contactsensitive device according to claim 12, wherein the member is in theform of a panel.
 17. A contact sensitive device according to claim 16,wherein the member has uniform thickness.
 18. A contact sensitive deviceaccording to claim 12, wherein the at least one sensor is mounted at anedge of the member.
 19. A contact sensitive device according to claim12, wherein the at least one sensor is mounted on the member spaced froman edge of the member.
 20. An active contact sensitive devicecomprising: a member capable of supporting bending wave vibration, anemitting transducer coupled to the member for exciting bending wavevibration in the member to probe for information relating to thecontact; at least one sensor coupled to the member for measuring bendingwave vibration in the member, and a processor operatively coupled to theat least one sensor for processing information relating to a contactmade on a surface of the member from a change in bending wave vibrationin the member created by the contact and measured by the at least onesensor and for applying a correction to convert the measured bendingwave signal to a propagation signal from a non-dispersive wave source.21. An active contact sensitive device according to claim 20, whereinthe emitting transducer is one of the group consisting of an inertialtransducer and a bender transducer, and the at least one sensor is theother of said group.
 22. An active contact sensitive device according toclaim 20, wherein the member is a display screen and wherein theemitting transducer comprises liquid crystals of the display screenwhich excite bending wave vibration in the member.
 23. A method ofdetermining information relating to a contact on a contact sensitivedevice comprising the steps of: providing a member capable of supportingbending wave vibration, generating bending wave vibration in the memberto probe for information relating to a contact, contacting the member ata discrete location to produce a change in the generated bending wavevibration in the member, measuring the changed bending wave vibration inthe member to determine a measured bending wave signal, and processingthe measured bending wave signal to calculate information relating tothe contact, including applying a correction to convert the measuredbending wave signal to a propagation signal from a non-dispersive wavesource.
 24. A method for use in a member capable of supporting bendingwave vibration and forming an acoustic radiator when excited, the methodcomprising: exciting bending wave vibration using an emitting transducercoupled to the member; measuring bending wave vibration in the memberrelating to a contact made on a surface of the member in at least onesensor coupled to the member; and processing information relating to thecontact measured by the at least one sensor and applying a correction toconvert the measured bending wave signal to a propagation signal from anon-dispersive wave source to identify information relating to thecontact.
 25. A method as recited in claim 24 further comprising causingthe member to produce an acoustic output using the emitting transducer.26. A method as recited in claim 24 further comprising isolating asignal generated by the contact from a signal generated by the emittingtransducer.
 27. A method as recited in claim 26 wherein the isolatingcomprises prediction filtering to predict a response of the signalgenerated by the emitting transducer.
 28. A method as recited in claim26 wherein the isolating comprises modeling the signal generated by theemitting transducer using continuous logging of the signal generated.29. A method as recited in claim 26 wherein the isolating comprisesusing multiple sensors to determine the location of the emittingtransducer to facilitate separation of bending waves generated by theemitting transducer from bending waves generated by the contact.